The present invention relates to a dry etching apparatus using reactive ions.
The IC has recently been more and more miniaturized, with the result that an ultra-miniaturized element having a minimum dimension of 1 to 2 .mu.m is now being tested. An ultra-miniaturized element such as this is made by using the so-called RIE (reactive ion etching) method. An etching gas such as CF.sub.4, for example, is introduced into an evacuated container which has parallel plate electrodes. A workpiece is mounted on the cathode electrode and a high frequency voltage is applied between the anode and the cathode electrodes, to generate a glow discharge between the electrodes. The positive ions in a plasma are accelerated by a cathode fall voltage generated on the surface of the cathode electrode, and vertically entered into the sample to physically and chemically etch the workpiece. However, with this RIE, which uses parallel plate electrodes, the glow discharge is comparatively poor in its gas dissociation effect. Thus, the SiO.sub.2 etching rate attained by using a CF.sub.4 +H.sub.2 gas, for example, is 300 to 500 .ANG./min, at most. It thus takes more than 30 minutes to etch an SiO.sub.2 film which is 1 .mu.m in thickness, which is extreme disadvantage in terms of mass production. For this reason a speedup of the etching rate is desired.
A dry etching method has been disclosed in Japanese Patent Disclosure (KOKAI) No. 56-161644, wherein magnets are arranged under the cathode electrode on which a sample or workpiece is mounted, and wherein etching is performed by moving said magnets. As shown in FIG. 1, electrons 5 are caused to undergo a cycloidal motion, by both a magnetic field 3 generated at a clearance 2 between the magnetic poles, which forms a closed loop; and by an electric field 4 generated between the electrodes, which field is perpendicular to this magnetic field 3. The collision frequency between the introduced reactive gas and the electrons is sharply increased by this electron motion, to generate a lot of reactive ions. These ions are entered vertically into a sample or workpiece 6, to achieve a high rate anisotropic etching.
However, the apparatus of this kind has the following drawback. Only track-like area 7, where magnetron-discharge of high density takes place, is etched when the magnetic pole clearance 2 remains unchanged. For the purpose of uniformly etching the whole of the workpiece 6, therefore, it becomes necessary to scan the magnetic pole clearance 2 more largely than the longer diameter of the workpiece 6. FIG. 2 is a characteristic curve showing the results which were obtained by measuring etching rate as a function of distances D from the edge of the workpiece 6, said etching rate being that found in the case of etching SiO.sub.2 by gas CF.sub.4. The magnetic pole clearance 2 was left unchanged, being separated from the edge of the workpiece 6 by 30 mm, as shown in FIG. 3. As may be seen from FIG. 2, etching of about 1000 to 2000 .ANG. was achieved for ten seconds, near the edge of the workpiece 6, and the etching rate became lower and lower, in approaching the edge of the workpiece 6. Even if the return of the scanning had been extremely fast, taking no more than 0.05 seconds, for example, it would have been tantamount to a case wherein the magnetic pole clearance 2 remained unchanged for two seconds, on both sides of the workpiece 6, in the course of about eighty time-scannings. Therefore, the depth of etching achieved at the edge of the workpiece, in the course of this number of scannings and under the conditions shown in FIG. 3, comes to have an approximate value of 500 .ANG.. This fast etching at the circumferential area of the workpiece is a cause which reduces the possibility of uniformly etching the whole of the workpiece. As a step toward the elimination of this cause, the magnetic pole clearance 2 might be scanned widely. In such a case, however, the apparatus would need to be made large-sized, thereby reducing the etching rate. This will be undesirable when the workpiece becomes larger (e.g., larger than 6 inches) in the future.